Tony Blair has warned the party faces electoral "annihilation" if left-wing MP Jeremy Corbyn is victorious in next month's leadership election.
The intervention came as Labour announced that over 600,000 people have applied for a vote in the party's leadership contest.
The party is struggling to deal with the enormous volume of new applications from people who have joined during a leadership election dominated by Corbyn.
Labour's membership stood at around 200,000 at the time of its general election defeat in May, but in the last three months around 400,000 extra people have signed up to the party in some form.
The influx includes 100,000 new members paying the full subscription rate, 121,000 applications from registered supporters paying a minimum of £3 for the right to vote in the leadership election, as well as 189,000 new affiliate members who are mainly trade union members.
This means that over 400,000 additional individuals are set to have the right to vote for the next party leader.
Around 100,000 are thought to have signed up in the final days of the leadership campaign, providing a massive boost to Corbyn, who has been urging his supporters to join up through campaigns on Twitter and Facebook.
The result is a complete change in the make-up of the people eligible to vote in the leadership election, with Labour party members set to be outnumbered by registered supporters and affiliate members.
Dozens of Labour party staff are currently working full-time to verify the new members.
The party has promised to weed out supporters who are not ideologically aligned with Labour and has already ejected hundreds of individuals who have previously stood for rival parties such as the Greens.
The party had to extend the deadline to register for the vote on Wednesday after its website crashed under the strain of new members joining at the last minute. Ballots will begin to be sent out on Friday, with the result announced on 12 September.
Meanwhile, some Labour politicians have already said that the result will lack legitimacy and will immediately be challenged in court due to problems associated with vetting the large influx of new supporters. Simon Danczuk, one of the party's MPs, also said there would immediate moves to oust Corbyn as leader if he wins the vote.
Jim Waterson is a politics editor for BuzzFeed News and is based in London.
Contact Jim Waterson at email@example.com.
Got a confidential tip? Submit it here.